The engine development process has often involved making decisions between competing engine characteristics, including fuel efficiency, power output, physical size, emission characteristics, reliability, and durability to name a few. In particular, emission characteristics are one criteria that are often evaluated by organizations like the Environmental Protection Agency (EPA). For instance, if some emission levels, such as nitrous oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) or particulate matter are too high for an engine, the engine may require expensive exhaust treatments such as a catalytic converter. In other instances, the engine might not be certified for operation or sale if it has poor emissions characteristics. As a result, engine emissions should be carefully considered during the engine development process. Some issues surrounding engine development regarding emissions characteristics are described below.
Carbon monoxide and NOx emissions (including both NO and NO2) are formed during combustion. Carbon monoxide generally results when combustion occurs with an air and fuel mixture that has more fuel than the stochiometric reaction requires (also known as a “rich” mixture). To address carbon monoxide concerns, most engines attempt to operate with stochiometric or lean (less fuel than stochiometric) air and fuel mixtures. However, some pockets of fuel rich zones will typically still exist in the air and fuel mixtures of conventional engines. These pockets can result in carbon monoxide production. Conversely, NOx emissions are high when the air and fuel mixtures are lean or near stochiometric values. Techniques used to address NOx formation often include the recirculation of exhaust gases into fresh air and fuel mixtures.
Among other causes, Hydrocarbon (HC) emissions can result from incomplete combustion or unburned fuel passing through a power cylinder during a period of intake and exhaust valve overlap. Cylinders of conventional engines often provide areas where it is difficult to sustain combustion, such as in the crevices between a piston and a cylinder wall. Additionally, most fuel injection systems cannot provide fuel that is completely evaporated before combustion begins. Fuel may also cling to the walls of a cylinder after it has been injected, forming a wet sheet of fuel that does not burn. This often leads to incomplete combustion in at least portions of a combustion chamber resulting in hydrocarbon emissions. Hydrocarbon emissions are often worse when an engine is first started, as the engines are typically cold and complete evaporation of fuel is difficult to support.
Both in compression ignition (diesel) and spark-ignition engines, the ratio of the fuel to air is not the same throughout the cylinder—thus not stochiometric throughout—due in part to poor mixing. Some part of the air and fuel mixture is fuel rich and some part is oxygen rich (i.e., lean). The crown of the piston (i.e., the top of the piston), the injection angle, and valve size and location, are some factors that are varied to control the flow of injected air and fuel mixture to improve mixing, however, they do not generally address the problem adequately. Non-stochiometric air and fuel mixtures may limit the maximum compression ratio of the engine, which controls the flame propagation speed and the combustion chemistry.
Another problem of conventional spark-ignition engines is knocking. Knocking limits the maximum compression ratio of conventional internal combustion engines and thus, the power efficiency of the engines. Knocking is a result of unwanted self-ignition or auto-ignition within the combustion chamber. To prevent knocking in spark ignition engines, it is most desirable to have a flame sheet that propagates from the ignition point outward at a high compression ratio. Due to the expansion of the gas behind the flame front, the unburned air and fuel mixture experiences high pressure and temperature before the flame front reaches the unburned region. When the pressure and temperature of the unburned air and fuel mixture are high enough, the mixture can self-ignite (i.e., auto-ignition), causing a rapid rise in pressure, which induces vibration of the cylinder walls and can create an audible knocking sound. This process is accelerated when there is enough time for sufficient auto-ignition precursors to form.
Two mechanisms control “knocking”: the formation of precursors and the temperature rise that accelerates the flame propagation rate. At high engine speeds, knocking may not be a problem since there is less time available for the precursors to form. On the other hand, as engine speed increases, there is less heat loss from the gases so that gas temperatures will be higher. This accelerates the precursor formation rate so that less time is required to form a concentration high enough for auto-ignition to occur. As a result of these two competing effects, some engines experience knocking at high speeds and some experience knocking at low speeds.
Knocking can be severe when the air and fuel mixture is at its stochiometric ratio. This problem has been solved in current engines in two expensive ways: the use of anti-knock additives and the lowering of the compression ratio. To prevent auto-ignition, high-octane fuel—a mixture of many hydrocarbons with high-octane additives—is used in high compression engines. If knocking persists even with the use of high-octane gasoline, it is eliminated by changing the ignition time to ignite the air and fuel mixture at a lower pressure (thus at a lower compression ratio) when the piston has moved downward from its highest position. However, this lowers fuel efficiency.
Conventional methods of developing products, and specifically internal combustion engines, often lead to lengthy development cycles and consequently high cost due to the iterative nature of such methods. For example, an engine designer may make a modification to one component of an engine which, in turn, requires him to make many other modifications to other, already designed and tested components of the engine. Making such changes may require re-evaluating the previously tested components, thereby adding cost and time to the development process.
The inventors of the present invention have found that the use of an axiomatic design approach offers a workable methodology to design an engine that addresses at least some of the above-mentioned issues. Using an axiomatic design approach can provide a process for designing an engine that allows one to achieve an engine with the characteristics they want by providing a clear description of how the designer can achieve these characteristics. Once the engine designer understands the design needs, the understanding is transformed into a minimum set of specifications, which are defined as functional requirements (FR's), that adequately describe “what the designer wants to achieve” to satisfy the design needs. The descriptor of “how the designer will achieve the needs” is articulated in the form of design parameters (DP's).
A basic postulate of the axiomatic design approach used to design the internal combustion engine described herein, is that there are fundamental axioms that govern the design process. In particular, there are two primary axioms associated with the axiomatic design approach.
The first axiom is called the independence axiom. It states that the independence of functional requirements (FR's) should be maintained, where FR's are defined as the minimum set of independent requirements that characterize the design goals. A set of FR's is the description of design goals. The independence axiom states that when there are two or more FR's, the design solution should allow each one of the FR's to be satisfied without affecting the other FR's. This means an engine designer has to choose a correct set of DP's to be able to satisfy the FR's and maintain their independence.
The second axiom is called the information axiom, and it states that among those designs that satisfy the independence axiom, the design that has the smallest information content is the best design. Because the information content is defined in terms of probability, the second axiom also states that the design that has the highest probability of success is the best design.
In summary, the independence axiom requires that the functions of the design be independent (i.e. decoupled) from each other, and not that the physical parts be independent. The second axiom suggests that physical integration is desirable to reduce the information content if the functional independence can be maintained.
Conventional internal combustion (IC) engines—both spark-ignition engines and compression ignition engines (e.g., diesel)—are coupled designs from the axiomatic design point of view. Ideally, the function of the product is specified in terms of functional requirements (FRs) and constraints (C), which are satisfied exactly as specified by choosing a correct set of design parameters (DPs). When a wrong set of DPs are chosen, a coupled design results. In a coupled design, the functional requirements (FRs) of a system—e.g., engine—are not independent from each other and therefore, each time a design parameter is changed to vary one of the FR's, other FR's change, making it difficult to satisfy all FR's within the desired range. Hence, in a coupled design, FR's must be compromised to achieve a minimally acceptable performance rather than making the system behave as originally envisioned and specified to achieve the ultimate results desired.
The basic causes for coupling are different between four-stroke cycle engines and two-stroke cycle engines, and also between spark-ignition and compression ignition engines. However, in current designs, the basic functions of these engines are coupled to each other and therefore, cannot be controlled precisely. In the case of most commonly used spark-ignition engines, fuel is injected using a fuel injector into the intake manifold or inlet port (port fuel injection) outside of the combustion cylinder, which evaporates and mixes with air and flows into the cylinder during the downward stroke of the piston in the cylinder. However, part of the fuel—either in vapor or liquid phase—remains in the manifold and does not combust in the cylinder. This unburned fuel is carried out of the intake manifold when the hot combustion product is exhausted from the cylinder. When the unburned fuel mixes with the hot exhaust gas, it partially oxidizes.
Further details of the axiomatic design approach as discussed herein can be found in “The Principles of Design” by Nam P. Suh, Oxford University Press, 198 Madison Avenue, New York, N.Y. (1990), and “Axiomatic Design, Advances and Applications” by Nam P. Suh, Oxford University Press, 198 Madison Avenue, New York, N.Y. (2001) both of which-are incorporated by reference in their entirety.